Variability in atmospheric chlorofluorocarbons (CCl₃F and CCl₂F₂) near a large urban area: Implications for groundwater dating

Chlorofluorocarbons (CFCs) are frequently used as tracers for age dating of young groundwaters. However, in urban environments with many CFC point sources, uncertainties in the delivery of CFCs to groundwater (input function) complicate quantitative interpretation of observed CFC distributions. To assess the potential impact of elevated atmospheric CFC mixing ratios on CFC-based groundwater ages near a large coastal urban center, we measured atmospheric mixing ratios of CFCs in Palisades, NY, 25 km north, of New York City. We present and discuss a 16-month record of atmospheric CCl₃F (CFC-11) and CCl₂F₂ (CFC-12) obtained from gas chromatographic analyses taken at intervals of approximately 10 min. Nearly all measured values are in excess of remote Northern Hemisphere (NH) atmospheric mixing ratios. The mean mixing ratios of CFC-11 and CFC-12 are 6 and 13% higher, respectively, than those measured at a remote NH location during the same time period. The temporal trends of CFC-11 and CFC-12 differ from those of the remote atmosphere. Diurnal, weekly, and seasonal patterns are evident in the measured CFC distributions, in addition to variations resulting from regional meteorological conditions. These observations indicate that, to effectively use CFCs as groundwater dating tools near local or regional sources, their local atmospheric input functions must be explicitly defined.